The present invention relates to a system for anodizing the surface layer of semiconductor materials such as Si or GaAs or superconducting materials such as Pb or Nb at a relatively low temperature with active species which are generated in a gas plasma containing oxygen.
A plasma anodization method is well known in the art as a method of oxidizing the surface of GaAs or Si at a relatively low temperature. This method is disclosed, for example, in Takuo Sugano, Semiconductor Plasma Technique, Sangyo Tosho, Tokyo (1980), pp. 83-91. According to one representative method, substances to be oxidized such as GaAs or Si are placed in an oxygen plasma which is generated by an RF (i.e., radio-frequency) or microwave power. According to this method of the prior art, however, the substances being oxidized have their surfaces irradiated with all active species (e.g., ions, electrons, excited atoms, excited molecules or photons) that are present in the oxygen plasma. Of these active species, moreover, most of the ions and electrons frequently irradiate the substrate with a high energy. As a result, the electrical properties of a MOS (i.e., metal-oxide-semiconductor) transistor, which is fabricated by making use of the interface between the oxide (e.g., SiO.sub.2) layer formed and the substrate (e.g., Si), are frequently inferior to those of the MOS transistor which uses the oxide film of SiO.sub.2 prepared by a thermal oxidation method.
Since the surfaces of the substances are exposed to various radiations from the plasma, moreover, it is difficult to control the oxidation parameter such as the temperature of the substances independently of the plasma generating conditions.
One of the plasma anodization methods of the prior art is disclosed in U.S. Pat. No. 4,232,057, as shown in FIG. 1. In this Figure: reference numeral 11 indicates a container; numeral 12 an oxygen atmosphere; numeral 13 an induction coil; numeral 16 a radio-frequency source; numeral 17 a plasma region; numeral 18 substrates; numeral 19 a holder; numeral 110 a heating source; numeral 111 a vacuum gauge; and numerals 112 and 115 indicate valves.
This discloses a technique that the control of a plasma power and the controls of the flow rate and pressure of oxygen are independent and that a uniform plasma-anodized substance can be formed by placing the substrates outside of the plasma region so that they can be provided independently of the temperature control. Despite of this fact, however, no consideration is taken into the influences due to the plasma.
There is also disclosed in Japanese Patent Laid-Open No. 56 - 37633 anodizing method which uses a plasma and a laser, as shown in FIG. 2.
In this Figure: reference numeral 21 indicates a laser source; numeral 22 a laser beam; numeral 23 a lens; numeral 24 a silicon substrate; numeral 25 a plasma generating chamber; numeral 28 a substrate table; numeral 29 an AC power supply; and numeral 210 a plasma generating coil.
According to this disclosure, the anodization is effected by instantly heating surface of the semiconductor substrate by the laser beam.
Since the heating is conducted by the laser beam, according to this method, the portions other than the heated portion can be left at a low temperature. However, this invites undesirable results because a thermal stress is caused between the portions heated by the laser beam and the other portions.